Time-lapse gravity and levelling surveys reveal mass loss and ongoing subsidence in the urban subrosion prone area of Bad Frankenhausen/Germany

Abstract

Abstract. We present results of a sophisticated, high-precision time-lapse gravity survey that was conducted over four years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Especially in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore, focus on the associated deformation processes. In this study, we combine levelling and highly precise time-lapse gravity surveys. Our investigation area is the urban area of Bad Frankenhausen in Central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravity and 18 levelling campaigns were carried out in quarter-yearly intervals within a local monitoring network. This network covers historical sinkhole areas, but also areas that are considered to be stable. Our results reveal ongoing subsidence of locally up to 30.4 mm a−1, with distinct spatio-temporal variations. Furthermore, we observe significant time-variable gravity changes in the order of 8 μGal over four years at several measurement points. In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the global soil water model GLDAS Noah to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity in urban areas for sinkhole investigations. Although the observed rates of gravity changes of 1–2 μGal a−1 are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks.

Investigation of the Ground Motion Near the Leaning Tower of Bad Frankenhausen Using Sentinel-1 Persistent Scatterer Interferometry

“…A precise levelling survey has been conducted since 1997/1999 by (Scheffler et al, 2013) and reveals irregular, but also decreasing movement rates of the church tower (~8 cm/yr between 2003-2005 and 3,5 cm/yr between 2010-2012). Furthermore, a high-precision time-lapse gravity and levelling survey was published by Kobe et al (2019), which also shows continuous subsidence in our study area due to subrosion-induced mass changes. To our current knowledge, no specific analysis of this subsidence process has been performed using SAR Interferometry yet, which makes this study even more interesting for complementing the gravity and levelling surveys.…”

Characterizing and monitoring a high-risk sinkhole in an urban area underlain by salt through non-invasive methods: Detailed mapping, high-precision leveling and GPR

“…The presented data illustrates that the simple and relatively inexpensive high-precision leveling geodetic method (Kersten et al, 2017;Desir et al, 2018;Kobe et al, 2019) provides highly accurate data for characterizing the vertical displacements associated with active sinkholes over relatively short monitoring periods. This approach is particularly suitable for urban areas with paved surfaces that facilitate the installation of benchmarks.…”

Analysis and Interpretation of Time-Lapse Micorgravity Data Case Study in Kota Lama Semarang Tourism Area

“…Gravity anomaly data will be used to estimate whether it area has experienced subsidence. This time-lapse microgravity data is used for the purposes mentioned above with the consideration that researches that have been conducted, both in Indonesia and in other parts of the country have been carried out, including [4], [5], [6], [7] and provide information that can be justified by comparing facts in the field.…”